In the area of electronic or electro-optical distance measurement, various principles and methods are known. One approach consists in emitting pulsed electromagnetic radiation, such as, for example, laser light, to a target to be surveyed and subsequently receiving an echo from this target as a back-scattering object, the distance to the target to be surveyed being determined on the basis of the transit time of the pulse. Such pulse transit time measuring devices have now become established as standard solutions in many areas.
In general, two different approaches are used for detecting the back-scattered pulse.
In the so-called threshold value method, a light pulse is detected if the intensity of the incident radiation exceeds a certain threshold value. This threshold value prevents noise and interfering signals from the background being incorrectly detected as a useful signal, i.e. as back-scattered light of the emitted pulse. What is problematic, however, is that detection is no longer possible in the case of weak back-scattered pulses, as are produced, for example, by relatively large measured distances, if the pulse intensity falls below the detection threshold. The substantial disadvantage of this threshold value method is therefore that the amplitude of the measured signal must be sufficiently greater than the noise amplitude of optical and electrical noise sources in the signal path, in order sufficiently to minimise incorrect detections.
The other approach is based on the scanning or the sampling of the back-scattered pulse. An emitted signal is detected by sampling the radiation detected by a detector, identifying a signal within the sampled region and finally determining the position thereof. By the use of a multiplicity of sampling values, a useful signal can also be identified under unfavourable circumstances, so that even relatively large distances or background scenarios involving noise or associated with interference can be handled. In the prior art, sampling is effected by scanning many identical pulses with shifting of the time window or of the phase, it currently being possible to realise very fast circuits which have a sufficiently high frequency to sample individual pulses. What is problematic, however, is the knowledge required beforehand about the approximate position, as a function of time, of the signal to be detected, since otherwise the time window as a period to be sampled and hence the data volume may be very large, or alternatively the use of many pulses and time windows to be shifted. However, a prohibitive disadvantage of signal sampling is that no appropriate information about the measured signal which can be evaluated is available in the state of saturated receiving electronics.
U.S. Pat. No. 6,115,112 discloses a measuring method by means of signal sampling, in which the time of arrival of the pulse is established approximately as a function of time by a coarse measurement carried out beforehand. The sampling is then effected as part of a precise measurement for a further light pulse, the limited possible period of arrival of which is now sampled. The measurement is thus divided into a coarse measurement and a precise measurement. The use of this approach inevitably demands a sequence since a time window in which the sampling measurements take place is defined only by the threshold value measurement. Thus, a sequence of coarse measurement and precise measurement on different pulses is effected separately as a function of time.
A substantial disadvantage of measuring principles known to date and based on the pulse transit time principle is therefore either the limitation of signal detection by a detection threshold or the necessity of establishing a time window for the sampling or the saturation of the detector.
Further disadvantages are the requirements regarding the technical components, such as, for example, large dynamic ranges, resulting from the limitation of these influences.